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Dark matter claims thrown into doubt by new data

By David Shiga

Previous claims that dark matter had been detected are being called into doubt now that a powerful new experiment has failed to see any of the elusive particles.

For years, scientists running the DAMA experiment beneath a mountain in Italy have reported a seasonal variation in the number of light flashes seen in their sodium iodide target. They claim the variability is due to changes in the speed of dark matter particles streaming through the laboratory as a result of Earth’s orbital motion around the sun.

CDMS-II, located in a Minnesota mine, is designed to detect dark matter particles by looking at the energy released when a particle smashes into a nucleus of germanium or silicon. Though the two candidate detections were unexpected, the researchers said they could not rule out the possibility that they were caused by interference from charged particles called cosmic rays or the decay of trace quantities of radioactive elements in materials used in the experiment.

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Now, a third experiment called Xenon 100, which its makers say is even more sensitive than the other two, has failed to detect the impact of any dark matter particles, casting doubt on the earlier results. “Dark matter particles continue to escape our instruments,” says Xenon 100 spokesperson Elena Aprile of Columbia University in New York.

Sensitive detector

Like DAMA, Xenon 100 is located in the underground Gran Sasso National Laboratory in Italy. But instead of sodium iodide, it uses liquid xenon as a target for dark matter particles.

Xenon is one of the heaviest elements in the periodic table. With three times the density of water, liquid xenon has many atoms per litre, maximising the chances that a dark matter particle will collide with it. If such a collision occurred, it would produce a flash of light that the experiment’s cameras would observe.

Xenon 100 collected data for 11 days in October and November 2009. A new analysis of the data submitted to Physical Review Letters, the Xenon 100 team reports that only uninteresting background radiation events were detected, with no sign of dark matter.

The power of the Xenon 100 detector means it should see dark matter events at a much higher rate than DAMA or CDMS-II, making up for its much shorter running time, says Xenon 100 team member Rafael Lang of Columbia University in New York.

Apples and oranges

If DAMA’s signal were due to dark matter, Xenon 100 would have seen dozens of events – unless the properties of dark matter are very different than expected, says Lang. “If DAMA really does see dark matter, it has to be something very, very exotic, something much different from what we expect, so it’s getting a bit unlikely.”

“The DAMA results are difficult to reconcile with other searches for dark matter, including this result,” says Jeter Hall, a CDMS-II member at Fermilab in Batavia, Illinois. But he adds&colon; “The detection technology and search strategies are very different.”

Rita Bernabei, spokesperson for DAMA at the University of Rome Tor Vergata, agrees and says the new data has no bearing on DAMA’s findings. “No direct model-independent comparison is possible among experiments which use different target materials and approaches and have different sensitivities to different dark matter candidates,” she told New Scientist.

Heavy dark matter?

If the CDMS-II detections are dark matter, the Xenon 100 results suggest the dark matter particles must weigh at least about 80 times the mass of a proton. That’s because CDMS-II is more sensitive to hefty particles than Xenon 100, which would have detected the particles if they weighed less than 80 protons, Lang says.

Xenon 100 started collecting more data in January. It now has more than 10 times as much data as obtained in the 2009 run, but the team has not yet started analysing it, Lang says.

“I think the results from Xenon 100 commissioning data are impressive,” Hall told New Scientist. “I am excited to see how far this experiment will advance the field when they release their full data set later this year.”